Abstract: The effects of several process parameters, such as substrate temperature, nucleation density, and substrate surface pretreatment, on the simultaneous formation of SiC and diamond under typical growth conditions of diamond by microwave plasma assisted chemical vapor deposition (MPCVD), have been investigated by scanning electron microscopy (SEM), Xray diffraction, and Raman and Fouriertransfer infrared (FTIR) spectroscopy. Results show that no SiC can be detected in the diamond films grown with a high nucleation density, whereas, SiC is detected in the thick diamond films grown with a low nucleation density, with or without surface pretreatment of the Si substrates. SEM micrographs and FTIR spectra illustrate that SiC is formed on the Si substrate not covered by diamond nuclei or in void regions between diamond nuclei. The formation of SiC and diamond on Si substrates under the growth conditions of diamond by MPCVD is a concurrent competitive deposition process, especially at the initial stage of diamond nucleation and growth. This is an alternative method for the synthesis of diamondSiC composites by MPCVD.